1. Field of the Invention
The present invention relates to an image forming apparatus, an adjustment method and a memory medium therefor. For example, an image forming apparatus capable of correcting color shift when forming a color image, a control method and memory medium therefor.
2. Related Background Art
Full-color image forming apparatus based on an electrophotographic process or ink jet process are becoming popular, and color shift is a factor contributing to deteriorating the quality of the image formed by such apparatus. A full-color image is usually formed by superimposing three or four color component images, and the positions of such color component images have to precisely matched on the recording sheet.
Also in the image forming apparatus of an electrophotographic process, there is known a configuration having image forming stations of different colors arranged in the conveying direction of the recording sheet, in order to achieve high-speed image formation. In such configuration, an error in the distance between the image forming stations results naturally in the color shift. Consequently, the color shift is corrected by controlling the timing of light emission of the exposure means for forming latent images of different colors. More specifically, in case an LED array is employed as the exposure means, the color shift correction is achieved, in the sub scanning direction, by the timing of light emission, and, in the main scanning direction, by displacing the LED to be used in the unit of each pixel.
Also in case of exposure means for forming the latent images by laser beam scanning, the method of correction is different according to the configuration of the polygon mirror to be employed. In a configuration having a polygon mirror for each color component, the color shift correction is achieved in the sub scanning direction by controlling the timing of light emission of the laser beam corresponding to each color and the phase angle of the polygon mirror, and, in the main scanning direction, by controlling the timing of start of latent image formation after the laser beam detection. On the other hand, in a configuration in which a polygon mirror is used commonly for different colors, the correction in the sub scanning direction is achieved in the unit of a pixel, by controlling the timing of light emission of the laser and changing the face of the polygon mirror to be used for latent image formation. Also the correction in the main scanning direction is same as in the type having plural polygon mirrors.
Also as the color shift is generated over the entire image area if the images of the different color components are mutually inclined, there is sometimes provided a mechanism for adjusting the inclination of the laser beam path with respect to the photosensitive drum.
On the other hand, there is widely employed the full-color image forming apparatus of an ink jet process in which an image is formed by a line or by a predetermined band width at a time, by reciprocating plural recording heads corresponding to the different color components in the main scanning direction. In such configuration, the color shift is generated if an error is involved in the distance of the recording heads corresponding to the different color components and in the position thereof in the sub scanning direction. In such case, the color shift correction is achieved in the main scanning direction by controlling the timing of start of recording of each recording head, and, in the sub scanning direction, by shifting the nozzle to be used for recording in the unit of a pixel.
The full-color image forming apparatus is provided, as explained in the foregoing, with adjustment means for absorbing the color shift resulting for example from the precision of component parts in the manufacture, and is shipped from the manufacturing site after adjustment so as to minimize the color shift. It is however difficult to always maintain the state of color shift after shipment, for example because of the following reasons:
(i) dimensional change dependent on temperature;
(ii) dimensional change resulting from distortion dependent for example on the location of installation;
(iii) change dependent on length of time;
(iv) kind of recording paper; and
(v) replacement of parts.
Following solutions have been proposed for the color shifts resulting from the above-mentioned causes:
(1) The color shift in the output image is read by an image reading device and is corrected. This method requires an image reading device, and is effective for example in a copying apparatus equipped with an image reading device, but cannot be applied to a single image forming apparatus not equipped with such image reading device;
(2) The color shift is estimated from the output image or the latent image in the image forming apparatus itself and is corrected according to the result of such estimation. This method however inevitably involves a higher cost, since there are required estimation means for calculating the result of estimation and correction means for correcting the color shift; and
(3) The operator recognizes the color shift in the output image and enters information into the image forming apparatus so as to minimize the color shift. This method can suppress the increase in the cost of the apparatus, but the operator is required to be capable of correcting recognizing even a slight color shift. In general, the recognizable shift of dots or lines formed on the recording sheet is 0.2 mm or larger, corresponding to a shift of about 5 pixels in a resolution of 600 dpi.
An object of the present invention is to provide an image forming apparatus capable of easily and precisely correcting the color shift, and an adjustment method and a memory medium therefor.
Other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description which is to be taken in conjunction with the attached drawings.